Lymphoma

Lymphoma in the central nervous system (CNS) can be classified as primary or metastatic. Primary lymphoma arises in the CNS and is usually a local disease. Metastatic lymphoma is found as an expression of systemic disease. Primary lymphomas tend to be parenchymal in origin whereas metastatic lymphomas are either dural-based or leptomeningeal and when also parenhcymal are usually associated with leptomeningeal disease. A striking response to steroid therapy is observed in CNS lymphoma leading sometimes to the disappearance of the lesion on imaging studies. Radiation therapy is also effective but overall prognosis is still poor. The best survival rates are those of primitive lymphoma displaying a single lesion, and are reported as long as 45 months.

Primary lymphomas are nearly all of the non-Hodgkin type. The site of origin remains controversial and unknown, since the CNS does not have endogenous lymphoid tissue nor a lymphatic circulation. Angiotropic lymphoma is a subtype. Primary lymphoma is rather infrequent in the immunocompetent population but it tends to affect immunocompromised patients and has now become quite common as a result of its frequent occurrence in the AIDS population. Reported median age at presentation is 60 years in immunocompetent and 33 in immunocompromised patients. Some differences have been observed in the appearance of the disease in the two groups and the differential diagnoses of the two groups are also different.

In general, the typical appearance of parenchymal lymphoma consists of masses that involve the deep grey matter structures, periventricular regions and corpus callosum. In particular the deep grey matter, although not the most frequent, is a quite characteristic site for primary lymphoma. Infratentorial locations are less common. The lesions are frequently multiple especially in the immunocompromized patients of whom up to 60% are thus affected. The infiltrative edges of the lesions extend along perivascular spaces and infiltrate blood vessel walls. Up to 75% of lymphomatous masses can be seen to be in contact with the ependyma, the meninges, or both. On CT masses are often iso- or slightly hyperdense, and on MR they tend to be iso- or hypointense as compared to grey matter on T2-weighted images. These signal characteristics are believed to reflect the high cellularity of the lesion. Usually, oedema is quite limited. On postcontrast images enhancement is seen most frequently with a homogeneous pattern. An enhancement confined to the peripheral rim (Fig.1), reflecting central necrosis (with a high signal on T2-weighted images), is uncommon in immunocompetent patients, at least before therapy, but involves about half the cases in immunocompromized patients. Haemorrhage and calcification are rare.

The differential diagnosis with the above mentioned imaging findings in immunocompetent patients includes several possibilities depending upon the appearance of the lesion. The diagnosis of lymphoma can be quite specific when it appears as a deep mass having a relatively homogeneous intensity similar to grey matter, dense contrast enhancement, and minimal oedema. If a focal-enhancing parenchymal mass is accompanied by enhancement along the perivascular spaces, especially with adjacent meningeal enhancement, the only diagnoses that should be seriously considered are lymphoma and sarcoidosis (see neurosarcoidosis). Other appearances are less specific and the differential diagnosis includes glioblastoma, metastases and other primitive tumours. The response to steroid therapy may help in the differential diagnosis element.

Important in the differential diagnosis of immunocompromized patients is toxoplasmosis. The imaging similarities are clear if the high incidence of multiple lesions and of ring enhancement of lymphoma in immunocompromized patients is taken into account. The single most valuable MR finding in distinguishing between lymphoma and toxoplasmosis in AIDS is the presence of subependymal spread, which is highly suggestive of lymphoma. Other features such as corpus callosal involvement, the nature of enhancement and increased density on plain CT, are helpful as differentiating features. None of them is, however, pathognomonic.

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